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algorithm: count letters (TheAlgorithms#1164)

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	`1`	`+/**`
	`2`	`+ * @function countLetters`
	`3`	`+ * @description Given a string, count the number of each letter.`
	`4`	`+ * @param {String} str - The input string`
	`5`	`+ * @return {Object} - Object with letters and number of times`
	`6`	`+ * @example countLetters("hello") => {h: 1, e: 1, l: 2, o: 1}`
	`7`	`+ */`
	`8`	`+`
	`9`	`+const countLetters = (str) => {`
	`10`	`+ const specialChars = /\W/g`
	`11`	`+`
	`12`	`+ if (typeof str !== 'string') {`
	`13`	`+ throw new TypeError('Input should be a string')`
	`14`	`+ }`
	`15`	`+`
	`16`	`+ if (specialChars.test(str)) {`
	`17`	`+ throw new TypeError('Input must not contain special characters')`
	`18`	`+ }`
	`19`	`+`
	`20`	`+ if (/\d/.test(str)) {`
	`21`	`+ throw new TypeError('Input must not contain numbers')`
	`22`	`+ }`
	`23`	`+`
	`24`	`+ const obj = {}`
	`25`	`+ for (let i = 0; i < str.toLowerCase().length; i++) {`
	`26`	`+ const char = str.toLowerCase().charAt(i)`
	`27`	`+ obj[char] = (obj[char] \|\| 0) + 1`
	`28`	`+ }`
	`29`	`+`
	`30`	`+ return obj`
	`31`	`+}`
	`32`	`+`
	`33`	`+export { countLetters }`

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	`1`	`+import { countLetters } from '../CountLetters'`
	`2`	`+`
	`3`	`+describe('CountLetters', () => {`
	`4`	`+ it('expect throws on use wrong param', () => {`
	`5`	`+ expect(() => countLetters(0)).toThrow()`
	`6`	`+ })`
	`7`	`+`
	`8`	`+ it('expect throws when using a number in the string', () => {`
	`9`	`+ expect(() => countLetters('h3llo')).toThrow()`
	`10`	`+ })`
	`11`	`+`
	`12`	`+ it('expect throws when using a special characters in the string', () => {`
	`13`	`+ expect(() => countLetters('hello!')).toThrow()`
	`14`	`+ })`
	`15`	`+`
	`16`	`+ it('count the letters in a string. Allows lower case', () => {`
	`17`	`+ const value = 'hello'`
	`18`	`+ const count = countLetters(value)`
	`19`	`+ expect(count).toEqual({ h: 1, e: 1, l: 2, o: 1 })`
	`20`	`+ })`
	`21`	`+`
	`22`	`+ it('count the letters in a string. Allows upper case', () => {`
	`23`	`+ const value = 'HELLO'`
	`24`	`+ const count = countLetters(value)`
	`25`	`+ expect(count).toEqual({ h: 1, e: 1, l: 2, o: 1 })`
	`26`	`+ })`
	`27`	`+`
	`28`	`+ it('count the letters in a string. Allows upper and lower case', () => {`
	`29`	`+ const value = 'HelLo'`
	`30`	`+ const count = countLetters(value)`
	`31`	`+ expect(count).toEqual({ h: 1, e: 1, l: 2, o: 1 })`
	`32`	`+ })`
	`33`	`+})`

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